Page 78 - Master Handbook of Acoustics

originally played and recorded at a higher level (assume 80 phons), you would need to increase both
bass and treble for proper balance. The loudness control found on some audio devices is intended to
compensate for the change in frequency response of the ear for different loudness levels by boosting
low and high frequencies. But the EQ curve corresponding to a given setting of the loudness control

might apply only to a specific loudness level of reproduced sound. This is an incomplete solution to
the problem. Consider the many things that affect the volume-control setting: The loudspeakers vary in
acoustical output for a given input power. The gain of power amplifiers differs. Listening-room
conditions vary from dead to highly reverberant. For a loudness control to function properly, the
system would have to be calibrated and the loudness control would have to adaptively vary the
frequency response relative to level at the listener. This is an example of how the ear’s nonlinear
level response introduces complexity to the tasks of recording and playback.

Area of Audibility

The results of tests administered to trained listeners are shown in Fig. 4-8. The listeners faced the
sound source and judged whether a tone at a given frequency was barely audible (contour A) or
elicited a sensation or feeling in the ears that may begin to be painful (contour B). These two contours
therefore represent the extremes of our perception of loudness.

FIGURE 4-8 The auditory area of the human ear is bounded by two threshold contours. (A) The